There are a number of different types of nuclear fuel in terms of what isotope is used, what other materials are mixed with the isotope and the physical configuration of the fuel. Nuclear reactors are designed to use a specific isotope in a specific shape

            The metal oxide form of uranium is used in many reactors because it has a higher melting point than pure uranium and it cannot burn. A series of chemical reactions is used to create uranium dioxide which is pressed into pellets and fired to create a dense solid material. This type of fuel is called UOX.

            Plutonium is blended with natural uranium or uranium that has already been depleted in fission processes to form a mixed oxide fuel called MOX. MOX is an alternative to the nuclear fuel used in most reactors and is a way to dispose of plutonium by transmutation.

            Metal alloys are also used for nuclear fuel. These include pure uranium as well as uranium alloyed with aluminum, zirconium, silicon, molybdenum. These fuels have the highest fissile atom density but cannot survive high temperatures like the oxide fuels.

            There is a nuclear fuel called TRIGA which is made of uranium zirconium hydride. As the temperature of the core increases the reactivity of TRIGA decreases which makes meltdowns improbable. This fuel is often used in reactors which produce neutrons for nuclear research.

            Uranium nitride is often used in NASA reactors and has the advantage of a very high melting point when compared to UOX. However, the best nitrogen isotope for this fuel is very expensive which reduces the desirability of this nuclear fuel.

            Uranium carbide was studied in the 60s and 70s but is not widely used as a nuclear fuel. It has a high thermal conductivity and a high melting point and the absence of oxygen makes this an attractive fuel. It may be the fuel of choice for the fourth generation reactors currently under development.

            Some liquid fuels have also been developed for use in reactors. The big advantage of liquid fuels is that they can be easily controlled but the big disadvantage is that they can easily leak out of the reactor if there is an accident. They are mixtures of lithium, beryllium, thorium and uranium fluoride salts used in molten salt reactors.

            All of the fuels discussed above are used in nuclear reactors. Radioactive materials can also be used in what are called atomic batteries. Plutonium-238, curium-242, curium-244 and strontium-90 have been used for this purpose. Atomic batteries can be non-thermal and use alpha and beta particles for energy generation or thermal which convert heat directly to electricity. Some thermal generators just use the heat from the decaying radioisotope.

            All these different fuels have their advantages and disadvantages based on their cost of production, thermal conductivity, neutron production, ease of handling, combustibility and a variety of other considerations.

            In order to control how much power is generated in a nuclear reactor, the number of neutrons available to cause nuclei to fission must be controlled. Control rods which absorb neutrons must be inserted into or withdrawn from the core in order to control the chain reaction.

            Less than half of the neutron flux in a nuclear reactor is produced by the emission of neutrons during the fissioning of uranium or plutonium. These neutrons are referred to as prompt neutrons. More than half of the neutron flux comes from the decay of radioactive isotopes that are the products of fission. These neutrons are called delayed neutrons. The half-life of the isotopes produced by fission range from milliseconds to minutes. In order to control the chain reaction, enough of the prompt neutrons must be absorbed by the control rods so that the critical mass necessary for the chain reaction will include the neutrons produced by the delayed neutrons. Otherwise, the reactor would go critical and melt down immediately.

            Neutron moderators can slow down the fast neutrons released by fission and convert them into thermal neutrons. Thermal neutrons are more likely to trigger more fission than fast neutrons. So increasing the moderation will increase the power output and lowering the moderation will lower the power output.

            In some reactors, the coolant functions as the neutron monitor so the power output can be increased by heating the coolant to make it less dense and less able to slow down the reaction. In other reactors, the coolant absorbs the neutrons and slows down the reaction. Heating the coolant will make it less dense and reduce its ability to absorb the neutrons so the power output will decrease.

            In emergency situations when the reaction must be stopped as quickly as possible or scrammed in nuclear terminology, a neutron absorber such as boric acid is injected into the reactor core automatically. If the automatic systems fail, there are manual backup systems that can inject the neutron absorber.

            In most types of reactors, there is a problem with the buildup of a radioactive isotope, Xenon-135 which is produced in the fission process. Xenon-135 absorbs neutrons slowing down or halting the chain reaction. If the reaction can be held at a high enough level, the xenon-135 can be destroyed as fast as it is produced. It is not always possible to keep the power output high enough and xenon-135 accumulation can be a serious problem.

            Iodine-135 is also produced by the fission process. It has a half-life of seven hours and will quickly decay into xenon-135. After a reactor is shut down, the iodine-135 will continue to decay into xenon-135. This xenon-135 will make it difficult to restart the reactor for a few days until it is transmuted into non-radioactive xenon-136 with the absorption of an extra neutron in each nucleus. The decline in the neutron absorption by the transmuting xenon-135 requires the control rods to be inserted deeper into the core to absorb more neutrons.

            Controlling a nuclear reactor is not a simple process. Each reactor design has its benefits and problems. Nuclear power generation is definitely a work in progress.

            A nuclear reactor is a complex device that is designed to start and sustain a controlled nuclear chain reaction. They are usually utilized to generate electrical power or to provide propulsion for ships and submarines. Controlled nuclear fission is used to heat either water or a gas which is then passed through a turbine. The turbine in turn either spins the propellers of a ship or the generators of an electrical power station.

            Uranium-235 and plutonium-239 are fuels that are used in nuclear reactors. When a neutron hits a nucleus of either, it may split into two smaller fragments that are nuclei of lighter elements. It also releases kinetic energy, gamma radiation and free neutrons.  Some of the released neutrons go on to hit other uranium or plutonium nuclei and cause them to fission releasing still more neutrons. This is known as a nuclear chain reaction.

            If the number of free neutrons continues to rise, a meltdown of the nuclear fuel can result. If the number of free neutrons falls off, the chain reaction will stop. In order to use nuclear materials for fuel in a reactor, it is necessary to keep the neutron flux in a certain stable range. This is accomplished by using something called a moderator to soak up free neutrons. Ordinary water is used to moderate about three quarters of the world’s reactors. Solid graphite is used in another fifth. And heavy water which contains extra neutrons in the nuclei of its hydrogen atoms accounts for most of the rest.

            Heat is generated in the reactor by several processes. The fast moving fragments from the fissioning of the uranium or plutonium collide with other atoms and heat them. Some of the gamma radiation is absorbed by the material of the reactor core and heats it. Some of the fission products are radioactive and will decay, release more heat.

            The heat generated in the reactor core is captured by water, a gas or a liquid metal. These substances are referred to as coolants. The heat absorbed by the coolant is used to generate steam which turns the turbines. Most reactors keep the coolant separate from the water that is heated to produce steam. However, the boiling water reactor directly heats the water that is used to power the turbines.

            There have been three generations of nuclear reactor put into operation so far. The fourth generation is currently being designed. Over the years, the reactors have been made more sophisticated with more automation, more backup controls, greater safety and greater reliability. These reactors have functioned well for the most part but every now and then extraordinary circumstances such as the tsunami in Fukushima have revealed short-comings which in turn lead to new designs.

            The stagnation of the nuclear power industry extended into the 1990s with few new plants being ordered and many ordered plants being cancelled. However, during the 90s a third generation of power plants were being designed to replace the second generation plants constructed in the 70s and 80s. This new design moved the moderator rods to a different location in the plant in order to minimize leaks. Plants with the new design were ordered in the late 90s.

            The Kyoto Protocols first signed by several nations in 1997 called for the reduction of carbon dioxide emissions by to 5% below the emissions in 1990. This demand for carbon dioxide reduction much of which comes from oil and coal fired power plants and the need to replace old second generation reactors which were scheduled for decommissioning led to pressure for the adoption of nuclear power. While nuclear power stations do not emit carbon dioxide, their construction involves huge quantities of cement which does emit a great deal of carbon dioxide.

            The election of George W. Bush, an enthusiastic supporter of nuclear energy to the US Presidency stimulated renewed interest in the expansion of nuclear power generation.             In 2004, Bush signed a resolution to allow the US Department of Energy to move forward on the construction of a long term nuclear waste depository at Yucca Mountain in Nevada. In 2005, Bush signed the first new US energy bill in more than a decade. The bill included funding for nuclear research, decommissioning of old nuclear power plants, tax credits and loan guarantees for the nuclear industry, and a liability cap in case of serious nuclear accidents. A number of consortia of companies were created to build more nuclear power plants.

            In the first decade of the Twenty First Century, design evolution continued with the development of new passive nuclear power plants that relied on natural processes such as gravity, convection, evaporation and condensation to achieve cooling instead of active systems of pumps. Emergency cooling water pools were built above the core so that in an emergency, the water could be easily dropped into the core. Other reactor designs relied on balls of graphite instead of graphite rods or water and were gas cooled.

            By 2007, there were over four hundred nuclear power reactions in the world in thirty one countries. France, Japan and the U.S. accounted for over fifty percent of nuclear generated electricity. As of 2010, China had over twenty five reactors under construction with plans to build many more. In the United States almost half of the operating reactors have had their licenses extended to sixty years.  

            Worldwide, there are currently four hundred and thirty six nuclear reactors generating three hundred and seventy gigawatts of electrical power. Another sixty three are under construction with a combined capacity of sixty gigawatts.

            The terrible nuclear accident at Fukushima where a tsunami caused by an earthquake destroyed four reactors a coastal nuclear power plant has caused many countries to reevaluate their reliance on nuclear power generation and their plans for expansion. Most of the power plants in Japan have been shut down for maintenance and then not restarted. Germany has decide to phase out all nuclear power plants by 2022 and Italy has banned nuclear power. The International Atomic Energy Commission has cut its estimated of new nuclear power generation by 2035 in half. However, in the United States, two new reactors were recently licensed for construction in Georgia and there are orders for more reactors.

            As nuclear power plants were built and produced power in the 1960s, local protests began to appear. Some scientists also started to raise concerns about nuclear power. There were fears of nuclear accidents, spread of nuclear weapons, cost overruns on power plant construction, terrorist use of nuclear materials and nuclear waste. By 1970, world production f nuclear power had reached one gigawatt or one billions watts).

             In the early 70s, big protests broke out against the construction of a nuclear power plant in Wyhl, Germany. The cancellation of the plant in 1975 inspired protests in other parts of Europe and in North America. The oil crisis in 1973 put pressure on countries such as Japan and France to find an alternative to oil fired power plants. They turned to nuclear energy as a substitute. Hundreds of thousands of people participated in multiple protests in France and Germany in the late 70s.

            In 1979, there was an accident at the Three Mile Island nuclear power plant in Dauphin County, Pennsylvania. Confusing controls and operator error led to the loss of large amounts of coolant which resulted in a partial meltdown of the fuel rods. This caused a release of small amounts of radioactive gases and iodine-131 into the environment.  For days, the owners of the plant and government authorities floundered as they tried to deal with the crisis. Poor communication with the public, confusion over the possible need for an evacuation and authorization of the release of fourty thousand gallons of radioactive waste water into the Susquehanna River undermined the credibility of the plant owners and the government with the public. An extended investigation led to the conclusion that there was no health danger from the accident. Clean up ultimately cost one billion dollars.

            World wide nuclear power production reached one hundred gigawatts by 1979. Rising construction costs due to construction delays and regulatory problems, dropping fossil fuel costs, public fears stoked by the Three Mile Island accident, law suits brought by groups opposing nuclear power and a lowering of demand for electrical power in the late 70s had major impacts on plant construction in the 70s. Sixty three nuclear power units were cancelled between 1975 and 1980.

            Nuclear power plant construction slowed in the 1980s due to the problems mentioned above. Many proposed plants were cancelled in the face of protests, law suits, rising costs and lower energy demands.

            In 1986, there was a terrible accident at the Chernobyl Nuclear Power Plant in Ukraine. A power surge triggered an attempt to run an emergency shut down which resulted in a greater power surge. This second surge ruptured a reactor vessel and exposed graphite moderator rods to the air. The resulting explosion and fire released large amounts of radioactive contamination into the atmosphere in the form of dust and smoke. The prevailing winds carried the radioactivity over much of the Western Soviet Union and Western Europe. Over three hundred thousand people were evacuated from parts of  Belarus, Russia and Ukraine. More that half of the radioactive fallout fell on Balarus.

            While the Chernobyl accident had a huge impact on public fear of nuclear power, it did not have a great effect on the regulation in Europe and the United States because the design of the Chernobyl reactor was a uniquely Soviet design.

            The World Association of Nuclear Operators was created in 1989 as a direct result of the Chernobyl accident to help the operators of nuclear power plants achieve the highest levels of safety and reliability.

            Where the 70s saw a huge increase in nuclear power generation, in the 80s the world capacity only tripled from one hundred gigawatts to three hundred gigawatts.

In the early Twentieth Century, it was discovered that radioactive elements could release great amounts of energy according to Albert Einstein’s famous equation, E = MC². This equation says that the energy in a quantity of matter is equal to that quantity of matter multiplied by the speed of light squared. There are approximately twenty five million kilowatt hours of energy in one gram of matter. Another way to think of it is that the amount of energy in one gram of matter is equivalent to the amount of energy released by burning five hundred sixty eight thousand gallons of gasoline.

            The early pioneers of radioactivity research did not believe that it would be possible to harness this vast amount of energy in matter as a power source. In the third decade of the Twentieth Century, it was discovered that neutrons could induce radioactivity disintegration called nuclear fission which resulted in the breakdown of uranium nuclei into roughly equal smaller nuclei. Because this reaction also released neutrons, it was possible to create a self-sustaining fission process called a chain reaction. The heat generated from this reaction could be used to boil water to generate steam and thus could be source of electrical power.

            In 1942, the first man-made reactor, called Chicago Pile-1, was created and became part of the Manhattan Project which went on to enrich uranium and build reactors to breed plutonium for the atom bombs dropped on Japan at the end of World War Two.

            After the War, the development of peaceful uses for nuclear power was advocated in the U.S. Some say that the U. S. government wanted to increase spending on nuclear weapons research and that the prospect of civilian use of nuclear power was just an excuse to get more money out of Congress. The fear that such research would create materials that could be used for bombs encouraged national governments such as the United States and the Soviet Union to keep all such research under strict government control. The United States Atomic Energy Commission was created in the U.S. to oversee nuclear research.

            The first use of nuclear energy to generate electricity was at Arco, Idaho on December 20, 1951 at the EBR-1 experimental station. The navy dedicated research to the development of a small nuclear reactor that could be used to power naval vessels. The USS Nautilus was the first nuclear powered submarine. It was launched in 1955. In 1953 Dwight Eisenhower, the U.S. President gave a famous speech titled Atoms for Peace at the United Nations that encouraged the international development of peaceful uses for nuclear power. The 1954 Amendments to the Atomic Energy Act allowed the declassification of U.S. reactor designs and encouraged construction of reactors by private industry. On June, 7, 1954, the U.S.S.R. started the first nuclear reactor which fed electrical power into a power grid at Obninsk.

            Advocates for civilian nuclear power stated that nuclear reactors would soon be producing energy at a cost that was comparable to conventional sources. There were also wild claims that nuclear power would soon be “too cheap to meter” or, in other words, free. There was some disappointment when this did not prove to be true.

            A United Nations conference on nuclear power was convened at Geneva in  1955. In 1957, EURATOM was created along with the European Economic Community which would become the European Union. The International Atomic Energy Agency was also launched in 1955.

            In 1956, England opened Calder Hall,  the world’s first commercial nuclear power plant in Sellafield. It started with a generation capacity of fifty million watts and later expanded to two hundred million watts. The Shippingport Reactor in Pennsylvania was the first commercial nuclear power plant in the United States. It was brought online in 1957. The age of atomic energy had begun.

            Unstable atomic nuclei undergo spontaneous decay involving the emission of gamma rays, electrons (beta particles), helium nuclei (alpha particles), protons and neutron. In this process, only a few elementary particles escape the nuclei of the decaying atom. There is another nuclear process that involves the decomposition of atoms. This process is called nuclear fission.

            In nuclear fission, an atom splits into two small fragments of roughly comparable size. As in radioactive decay, fission often produces gamma rays, electron, protons, and neutrons. It also releases a huge amount of energy when compared to decay processes. That energy can be harnessed to create nuclear power or nuclear explosions.

            There are natural radioisotopes that undergo fission when struck by a neutron. These radioisotopes are call fissile and some can sustain a process known as a chain reaction when neutrons emitted by one of their atoms causes another atom to fission. The second atom releases a neutron when it fissions and causes a third atom to fission. Thus, like links in a chain, the reaction proceeds from atom to atom. Some natural isotopes such as uranium-235 and plutonium-239 are called nuclear fuels because if they are present in a high enough concentration, they can sustain a chain reaction. They can be harnessed to generate power or nuclear explosions. The amount of energy released in given amount of fissile materials is millions of times greater than the same amount of other common fuels such as coal, oil and gasoline.

            Free neutrons sustain a chain reaction. If the number of free neutrons in a fissile material declines, the chain reaction will stop. If the number of free neutrons in a fissile material increases, the reaction will accelerate and run away. The nuclear material either will heat up to the point that it will melt through any container and sink into the soil or there will be a nuclear explosion. If the number of neutrons can be controlled and kept at a constant level, then the sustained reaction can be used as a source of power. Some substances such as water can absorb neutrons and can be used to moderate a chain reaction. These substances are referred to as moderators.

            About 2 billion years ago, the percentage of uranium-235 in natural uranium deposits was three percent, much greater than the seven tenths of one percent that it is today. There were uranium deposits in Gabon, Africa at Oklo that were concentrated enough that the uranium-235 could sustain a chain reaction. The ground water moderated the chain reaction and kept it from running away and the deposits generated about one hundred kilowatts for several hundred thousand years. So the first nuclear reactors that produced sustained power were actually created by natural processes in Gabon.

            The biological damage of ionizing radiation occurs at the level of the cell. Radiation can tear apart the DNA in the nucleus of the cell. Most breaks in DNA are repaired within twenty four hours but twenty four percent of those repairs are incorrect. If the damage is severe enough, it will kill the cell in a process called apoptosis where the cell go through a process of programmed cell death. If the damage to DNA does not kill the cell, it can cause changes in the DNA sequence known as mutations. These can be passed to daughter cells when the damaged cell divides and cause cancerous tumors. Since the DNA contains the recipe for all the proteins in the human body as well as instructions for the manufacture of proteins, abnormal proteins or abnormal protein production can be the result leading to premature aging and cancer.

            Natural radiation produced by naturally occurring radioactivity in the human body produce a low rate of DNA damage and mutation. X-rays, gamma rays, beta particles, alpha particles and neutrons can all produce DNA damage and mutations at a much greater rate.  Experiments have shown that a given amount of radiation spread out over time in repeated exposures can have a more damaging effect than the same amount of radiation given in a single high dose.

            Radiation damage to cells causes the release of free radicals of oxygen which are highly reactive and also cause biological damage. The oxygen can damage the bases which make up DNA as well as the enzymes that repair damaged DNA. Radiation damage can be passed to neighboring cells in human tissue. Different types of radiation cause different amounts and types of damage.

            Alpha particles or helium nuclei are very energetic and produce a great deal of damage but it is highly localized in tissue. It can cause 20 times the damage that x-rays and gamma rays can cause.

            Beta rays or energetic electrons are more penetrating than alpha particles but cause less damage.

            Neutrons can cause damage in a number of ways. They can cause other elements to become radioactive. They can release free radicals that damage molecules. They can cause protons to be ejected from nuclei. These protons directly damage tissue.

            X-rays and gamma rays can pass through tissue more easily than alpha and beta particles and the damage they cause is more evenly distributed in tissue. X-rays can also cause electron or beta particles to be ejects from stable atoms.

            Cancers associated with large doses of ionizing radiation include leukemia, thyroid, breast, bladder, colon, liver, lung, esophagus, ovarian, melanoma and stomach cancer. The latent period refers to the time between exposure to radiation and the detection of cancer. The cancers caused by ionizing radiation are indistinguishable from cancers caused by other carcinogens such as cigarettes, alcohol, harmful foods, cleaning products, and industrial pollutants in air, soil and water.

            Radiation hormesis is an unproven theory that exposure to low levels of ionizing radiation may help to immunize the body against damage from higher levels of ionizing radiation. The mechanism is thought to be based on activation of the DNA repair machinery by the low level radiation.

            Radioactivity is definitely a threat to our health. It has been said that there is no safe minimum dose of radiation but we seem to survive in a natural environment with many different sources of radiation both outside and inside our bodies. On the average, there are over 800 radioactive events in the human body every second. Here is a list of the radioactive isotopes in our bodies.

(There are 28.3 grams in one ounce, one thousand milligrams in a gram, on million micrograms in a gram, one billion nanograms in a gram, one trillion picograms in a gram, and one quadrillion femto grams in a gram, and one thousand grams in a kilogram)

Potassium-40 – 16.5 milligrams – 4,340 disintegrations per second

Carbon-14  – 16 nanograms – 3080 disintegrations per second

Rubidium-87 – 190 milligrams – 600 disintegrations per second

Lead-210 – 5400 picograms – 15 disintegrations per second

Helium-3 – 20 femtograms – 7 disintegrations per second

Uranium-238 – 100 micrograms – 3-5 disintegrations per second

Radium-228 – 46 femtograms – 5 disintegrations per second

Radium-226 36 micrograms – 3 disintegrations per second

            The uranium, potassium and rubidium in our bodies were created in stellar explosions before the Earth was formed. The lead and radium isotopes were created by thorium and uranium decay. Helium-3 and Carbon-14 are being continuously created by cosmic rays bombarding the atmosphere of the Earth.

            Potassium-40 is present in all the food that we eat in tiny quantities. Potassium is abundant in our environment and plants take it up from the soil. We consume about two and one half grams of potassium every day. It is an essential part of our diet and our bodies maintain a constant level.

            Carbon-14 makes up a tiny amount of the roughly 16 kilograms of carbon in our bodies. It is constantly being created by cosmic rays interacting with nitrogen in the atmosphere. All living things breath in tiny amounts of carbon-14 as their bodies constantly replace carbon. Carbon-14 can be used to date the age of a biological material because when something dies, it stops taking in carbon-14 which decays and slowly disappears.

            Rubidium has no known biological function but probably mimics potassium in our tissue. Rubidium is a very abundant element in the Earth’s crust but rubidium-87 makes up a very small part of all the rubidium.

            Most of the lead-2190 in our bodies comes from the food we eat but some of it is inhaled from the air as a decay product of Radon-222. People also absorb lead-210 from smoking cigarettes.

            Helium-3 or tritium is present in the water we drink in minute quantities. Originally all the helium-3 was produced by cosmic rays interacting with nitrogen in the air but since the advent of nuclear power, a small amount of helium-3 is release from nuclear power plants in water vapor.

            Uranium-238 finds its way into our bodies as a contaminant in the food that we eat. Its decay generates other radioisotopes.

            Radium is present in all soils and in the water in some areas. The radioisotopes of radium are present in all the food that we eat and some of the water we drink depending on the area we live in. Radium 228 and Radium-226 decay into other radioisotope which emit radiation as they decay.

            There are many other radionuclides in our bodies but they do not contribute much radiation. We may also have iodine-131, cesium-137 and strontium-90 from fallout from nuclear explosions and some will have radioisotopes from nuclear medicine. Radon is always present in the air we breathe in small amounts.

            It is difficult to pin down the amount of damage these radioisotopes do in the body. Different isotopes mimic different elements that cause different organs to absorb them. Iodine-137 is taken up by the thyroid gland, cesium-137 winds up in muscle tissue and strontium0-90 and radium-228 and 226 make their way into the bones. Different people have different sizes of organs, different amounts of muscle and different mass in bone. In the bones, a lot of alpha particles from radioisotopes hit bone and do no damage. Others hit cells and kill them. So the actual danger of an alpha particle causing cancer is very low compared to all the alpha particles emitted.

            A variety of radioisotopes is present in our environment and in our bodies. It is part of our life and inescapable. Most of it does no damage but a small amount accounts for different illnesses, many of which can be fatal.

            Radioactive contamination of food has become a big concern since the Fukushima. This articles will explore how radioactivity finds it’s way into the food chain.

            Plants can take up radioactivity from the naturally occurring radioisotopes in the soil and from man-made radioisotopes from nuclear testing and releases from nuclear power plants. Soil contains natural uranium-238 and uranium-235, thorium-232, potassium-40, radium 226 and radon 222. Iodine-131, cesium-137 and strontium-90 come from man-made sources.

            Where food is gathered from a natural environment such as mushrooms in a forest, the radioisotopes are easily taken up by the plant and fungi from the soil. Radioisotopes are more tightly bound to soil particles in agricultural regions and is less easily absorbed by growing plants. The type of soil used to grow food plants is important when considering absorption of radioactivity. In soil that has a lot of clay and low amounts of organic material, cesium-137 is locked up and immobilized by clay particles. In other soil where the concentration of clay is low and there is a lot of organic material, radioisotopes are mobile and readily available for absorption. Greater acidity of soil also favors the absorption of radioactivity by plants.

            In plants such as red lettuce and other vegetables where the surface area is big compared to the weight, radioactive dust particles carried by winds and washed out of the air as rain can contribute to the contamination of food by radioactivity.

            Milk and other dairy products are in danger of significant radioactive contamination when cattle eat grass that has been exposed to fallout in the wind and rain. The fact that cattle may consume a large amount of contaminated grass every day  leads to concentration of radioisotopes such as strontium-90, iodine-137 and cesium-137.

            Animals such as pigs and cattle as well as birds such as chickens which are raised for meat can absorb radioactivity from their feed as well as directly from fall out. Cesium-137 tends to concentrate in muscle meats and is a special concern. Strontium-90 will accumulate in bones and is not as dangerous unless the bones are ground up and use in food products.

            Ocean water contains a small amount of natural radioactivity. The main concern with seafood is radioisotopes resulting from human activity such as Iodine-131, cesium-134 and cesium-137. The iodine is water soluble and has a short half-life. The cesiums have half-lives in years and can react or be picked up by particles. They are either suspended in the water or fall to the bottom. Cesium builds up in bottom feeding bivalves such as clams, oysters and gooey ducks. They are also consumed by crustaceans such as crabs and lobsters. The bivalves are either directly consumed by people or by larger fish which are consumed by people.

            Iodine-131 is mainly a problem in the first few days after a nuclear accident because of its water solubility and short half-life. However, if exposed food is consumed in that short time period, the iodine-131 will concentrate in the thyroid gland and may lead to thyroid cancer.